Two-stage purification of fibrous cellulose material employing gaseous chlorine dioxide in one stage and a peroxygen compound in the other

ABSTRACT

A two stage process for bleaching an unbleached chemical cellulosic pulp in which the first stage comprises fluffing a pulp that has been adjusted in moisture content to a consistency of from about 20 to 60 percent, contacting the pulp with gaseous chlorine dioxide diluted with a non-reactive gas at a temperature of between 15* and 100* C. for a period of 20 seconds to 60 minutes, the pulp then being washed with water; and finally in the second stage adjusting the pulp moisture content to 5 percent and contacting the pulp with a peroxygen compound having a peroxygen content of from 0.025-1.5 percent by weight based on the weight of the dry pulp at a temperature up to 100* C. for a period of from 1/2 to 5 hours.

United States Patent Yorston et al.

[is] 3,655,505 [451 *Apr. 11, 1972 TWO-STAGE PURIFICATION OF FIBROUS CELLULOSE MATERIAL EMPLOYIN G GASEOUS CHLORINE DIOXIDE IN ONE STAGE AND A PEROXYGEN COMPOUND IN THE OTHER Inventors: Frederick H. Yorston, Montreal; Norman Liebergott, Chomedey, Quebec, both of Canada Pulp and Paper Research Institute of Canada, Pointe Claire, Quebec, Canada The portion of the term of this patent sub sequent to June 22, 1988, has been disclaimed.

Filed: Dec. 12, 1969 Appl. No.: 884,749 v Related U.S. Application Data Continuation-in-part of Ser. No. 559,714, June 23, 1966, abandoned.

Assignee:

Notice:

Foreign Application Priority Data June 25, 1965 Great Britain ..27,158/65 U.S. Cl ..l62/67, 8/108, 162/78,

162/88 Int. Cl ..D21c 9/14 Field of Search 1 62/67, 78, 88, 100; 8/l08.5

References Cited UNITED STATES PATENTS 3,020,197 2/1962 Schuber ..162/78 2,730,426 1/1956 BcckeL. 162/88 X 3,345,250 /1967 Brinkley et ....l62/88 X 3,433,702 3/1969 Jack et al 162/87 2,516,384 7/1950 Hill et a1. ..162/100 3,316,141 4/1967 Bergholm et a1 162/100 OTHER PUBLICATIONS Becco Chemical Division (Research and Development), Becco Procedures for Bleaching Kraft Pulps," Bulletin D- 3, Food Machinery & Chemical Corp. 1955).

Casey, Pulp and Paper, Second Edition, lnterscience, NY. (1960) Vol. 1 p.506.

Rydholm, Pulping Processes, lnterscience, NY. (1965) pp. 973-974.

Primary Examiner-S. Leon Bashore Assistant Examiner-Richard H. Tushin Attorney-Stevens, Davis, Miller and Mosher [57] ABSTRACT A two stage process for bleaching an unbleached chemical cellulosic pulp in which the first stage comprises fluffing a pulp that has been adjusted in moisture content to a consistency of from about 20 to 60 percent, contacting the pulp with gaseous chlorine dioxide diluted with a non-reactive gas at a temperature of between and 100 C. for a period of seconds to minutes, the pulp then being washed with water; and finally in the second stage adjusting the pulp moisture content to 5 percent and contacting the pulp with a peroxygen compound having a peroxygen content of from 0.025-1.5 percent by weight based on the weight of the dry pulp at a temperature up to C. for a period of from A to 5 hours.

5 Claims, No Drawings TWO-STAGE PURIFICATION OF FIBROUS CELLULOSE MATERIAL EMPLOYING GASEOUS CHLORINE DIOXIDE IN ONE STAGE AND A PEROXYGEN COMPOUND IN THE OTHER This application is a continuation-in-part of application, Ser. No. 599,714, filed June 23, 1966, now abandoned.

The present invention relates to the bleaching of a chemical cellulosic pulp and in particular to the bleaching of an unbleached chemical cellulosic pulp, such as wood pulp produced for example in the Kraft or sulfite process to produce a semior fully-bleached chemical cellulosic pulp suitable for newsprint furnish or as semi-bleached market pulp.

Commercially bleached chemical cellulosic pulp is produced by subjecting the pulp e.g. the Kraft or sulfite pulp, at low consistency to a multistage bleaching process involving at least three stages. In particular in such a multistage process the pulp from the digestion stage of the mill after washing is first treated with chlorine dioxide at low consistency to effect solubilization in water and caustic soda of ligneous impurities in the pulp. This chlorination is effected on the pulp at low consistency, usually of from 2.5 to 3.5 percent as an aqueous slurry for a time varying from 30 to 90 minutes. The chlorinated pulp is then washed with water to remove the water soluble impurities therefrom and extracted with caustic soda to remove the alkali soluble ligneous impurities therefrom. Again, the extraction with caustic soda is effected on the pulp at a relatively low consistency to 14 percent i.e. as an aqueous slurry for a period of from 60 to 120 minutes at a temperature in the range 60 to 80 C. The extracted pulp is washed and oxidized usually with chlorine dioxide or with hypochlorite to remove further lignin therefrom and the oxidized pulp again washed to produce the bleached chemical cellulosic pulp. In the oxidation stage, using hypochlorite the pulp is treated with the hypochlorite as an aqueous slurry at a relatively low consistency of 10 to 15 percent at a temperature from 35 to 45 C. and a retention time of 150 to 180 minutes. Using chlorine dioxide, the temperature is in the range 65 to 85 C. and the retention time is from 180 to 300 minutes. When the pulp bleached is a sulfite pulp the aforesaid three stages are sufficient to produce a fully bleached pulp and with a Kraft pulp-a semi-bleached pulp is produced which is usually subjected to further caustic extraction stages and/or chlorine dioxide oxidation stage with intermediate washings to produce a fully bleached pulp.

The aforesaid three stages in the multistage bleaching of unbleached chemical cellulosic pulp are time consuming and inefiicient. In particular, the aforesaid three stage procedure involves a substantial expenditure in capital equipment to handle the material, a substantial expenditure in heat and thus large operating costs and a substantial expenditure in labor.

The present invention provides a two stage process which replaces the aforesaid three stages in the aforesaid multistage bleaching process, particularly with Kraft pulp which two stage process is effected in a substantially shorter time and which may be effected much more economically with lower expenditures in capital equipment operating costs, costs of materials and cost of labor to effect the bleaching of the unbleached pulp and produce a bleached pulp of comparable brightness and quality to the pulp produced in the first three stages of the conventional multistage process.

It has now been found according to the present invention that a bleached pulp of comparable brightness and 0.5 percent CED viscosity to the pulp bleached in the first three stages of the conventional multistage bleaching process is achieved by effecting the bleaching in two stages, namely by treatment of the unbleached pulp at high consistency and in flufied or shredded form with gaseous chlorine dioxide and after an intermediate washing of the pulp with water, treatment of the washed pulp with a peroxygen compound.

According to the present invention there is provided a process for bleaching an unbleached chemical cellulosic pulp which consists essentially of two stages the first stage comprising adjusting the moisture content of said pulp so that the pulp has a consistency from about 20 to 60 percent, fluffmg or shredding the pulp into fibers and fiber aggregates, contacting said fluffed pulp with chlorine dioxide diluted with a non-reactive gas to a partial pressure less than millimeters of mercury at a temperature between 15 and 100 C. for a period of from 20 seconds to 60 minutes and washing said treated pulp with water and the second stage comprising adjusting the moisture content of said washed pulp such that said pulp has a consistency from 5 to 50 percent and contacting said pulp with a peroxygen compound ina peroxygen content of 0.025 to 1.5 percent by weight based on the weight of the dry pulp at a temperature of up to C. for a period of time from &2 to 5 hours.

The process of the present invention thus consists essentially of two stages, namely a first stage in which the unbleached pulp at high consistency and in fluffed and shredded form is treated with gaseous chlorine dioxide to effect a gaseous phase reaction and after intermediate washing with water the oxidized pulp so obtained is treated with the peroxygen compound to produce the bleached pulp. In particular the two stage process of the present invention replaces the first three stages in the conventional multistage bleaching of an unbleached chemical cellulosic pulp and while with a sulfite pulp a fully bleached pulp will be obtained, with a Kraft pulp a semi-bleached pulp will be obtained and such semi-bleached pulp can be subjected to further bleaching stages to obtain a fully-bleached pulp by subsequent treatment with chlorine dioxide and caustic extraction stages as in the conventional multistage bleaching process. Thus in general while the process of the present invention consists essentially of the aforesaid two stages, it is also within the scope of the invention that subsequent stages may be effected if desired.

It is critical to the present invention that the pulp in the first stage of the process of the present invention i.e. the stage in which it is treated with chlorine dioxide, is at high consistency in the range 20 to 60 percent and that the pulp is in fluffed or shredded form. By observing these conditions the oxidation of the ligneous impurities in the pulp is effected by a gas phase reaction with the gaseous chlorine dioxide in intimate contact with the individual fibers of the pulp which provides for a very fast oxidative action of the gas to react with the pulp and thus allows a very short contact time of generally at most 60 minutes depending upon the brightness required in the product pulp. The contact time may be as low as 20 seconds although the contact time is usually of the order from 3 to 10 minutes. With the aforesaid conditions, the pulp obtained after subsequent treatment with the peroxygen compound has a comparable brightness and other qualities to the pulp obtained from the first three stages in the conventional multistage bleaching process.

As used herein the term fluffed or shredded pulp" is the product obtained by the fragmentation of a chemical cellulosic pulp into fibers and fiber aggregates or flocs, substantially each discrete fiber aggregate or floc being spongy, compressible and readily gas permeable. Each of said fiber aggregates or flocs is usually an average of V8 to 1 inch diameter and presents a fluffed up dry appearance, even with about 70 to 80 percent moisture in the pulp. Thus reference to fluffing or shredding of chemical cellulosic pulp is directed to a process for producing this pulp by fragmentation. ln order to fully realize the advantages of the gas phase reaction between the chlorine dioxide and the high consistency pulp in the multistage bleaching of fibrous chemical cellulosic pulp the comminution of the pulp i.e. the shredding or fluffing of the pulp should be of a specific nature to produce fragments which, independently of their size, are of low density, porous structure throughout and substantially free from any highly compacted portions, i.e. compacted fiber bundles. By operating under these conditions, it is possible to obtain a relatively high speed process in which the fluffed pulp is contacted with the gaseous reagent over a substantially reduced period of time as compared with the conventional multistage bleaching process and which involves the use of substantially reduced volumes of water for evaporation. In particular by adjusting the consistency of the pulp to be of high consistency, and by contacting the chlorine dioxide in gaseous form with the pulp shredded or fluffed into fiber or fiber aggregates, it is possible to obtain a fast intimate contact of the chlorine dioxide with the interior of the fibers or fiber bundles and, as a result, to reduce the time necessary for obtaining the extent of reaction necessary for high brightness in the pulp. Due to the speed of the process, the retention time in continuously operated reactions is greatly reduced which allows the use of small equipment of lower capital cost. Therefore the presence of smaller quantities of pulp in the process makes for more precise control of the operation and furthermore, the sharp reduction in the quantity of water involved in the use of high pulp consistency results in a substantial reduction of heat input to the reaction and correspond saving. Again, it is found generally that there is a saving of the chlorine dioxide over and above that necessary in conventional chlorine dioxide bleaching stages. By operating the first chlorine dioxide stage according to convention techniques with a low consistency pulp slurry the advantages of the present invention would not be achieved.

It is also essential to the first stage of the process of the present invention that the chlorine dioxide be diluted to a partial pressure of less than 100 millimeters of mercury with anon-reactive gas such as air, nitrogen or steam as above a partial pressure of 100 millimeters of mercury there is a hazard of explosion of the chlorine dioxide.

The temperature of the reaction in the first stage of the process of the present invention is suitably between 15 and 100 C. and more preferably in the range 30 to 60 C. In general the reaction of the chlorine dioxide gas with the high consistency pulp is an exothermic reaction and a substantial proportion of the heat required for maintaining the reaction temperature is supplied by the reaction necessitating a minimum of heating.

After washing the oxidized pulp it is further oxidized directly at a second stage by treatment of the pulp again at a consistency in the range to 50 percent, suitably in the range to 30 percent with the peroxygen compounds such as sodium peroxide, benzoyl peroxide, chloracetic acid, hydrogen peroxide or ozone in a peroxygen content of 0.025 to 1 percent based on the dry fibers and at a temperature of up to 100 C. for a retention time of /2 to 5 hours. By the combination of the peroxygen treatment of the chlorine dioxide oxidized pulp it is found that in a simple economic manner one can substitute the initial three stages of the conventional multistage bleaching process and obtain a bleached pulp comparable in quality and brightness to that obtained in the first three stages of the aforesaid multistage process.

The present invention will be further illustrated by way of the following examples.

EXAMPLE I An unbleached Kraft pulp from coniferous wood having a Roe chlorine number of 3.97, a Kappa number of 23.5 and a 0.5 percent CED (cupriethylenediamine) viscosity of 23.7 was pressed to 30 percent consistency, divided into fibers and fiber aggregates by passing through a disc mill and placed in a glass vessel located in a bath maintained at 60 C. Chlorine dioxide diluted with nitrogen to a partial pressure of 30 mm. of mercury was passed into the vessel until 1.8 percent of chlorine dioxide on the weight of dry fiber had passed through the fluffed pulp. The pulp was held at 60 C. for 10 minutes and from the time the chlorine dioxide-nitrogen mixture was applied to the pulp. Unreacted chlorine dioxide was swept away from the vessel by a stream of air and absorbed in cold water. Consumption of chlorine dioxide was 1.44 percent of the weight of dry fiber. The pulp was then washed. A portion of it was treated at percent consistency with 1.5 percent of its weight of sodium peroxide for 1 hour at 60 C. Characteristics of the semi-bleached pulp were: brightness 68.4 and CED viscosity 21.9 cp.

A second portion of the same treated pulp was treated with 1.5 percent of its weight of sodium peroxide for 3 hours (to substantial exhaustion of peroxide) at 60 C. Characteristics of the semi-bleached pulp were brightness 71.6 and CED viscosity 22.5 cp.

EXAMPLE ll Fluffed samples of the same Kraft pulp as in Example I at 30 percent consistency and at room temperature were exposed to chlorine dioxide diluted with nitrogen to a partial pressure of 30 mm. of mercury. The temperature rose to 32 C. Retention time from the time the chlorine dioxide-nitrogen mixture was applied to the unbleached pulp was 3 minutes. Consumption of chlorine dioxide was 1.02 percent of the dry weight of fiber. The pulp was then washed. The pulp was then treated at 20 percent consistency with 1.5 percent of the weight of sodium peroxide for 3 hours at 60 C. Characteristics of the semibleached pulp were brightness 60.4 and CED viscosity 27.8 cp.

EXAMPLE Ill Fluffed samples of the same Kraft pulp as in Example 1 at 60 percent consistency and at room temperature were exposed to a mixture of chlorine dioxide and nitrogen as in Example I. The temperature rose to 42 C. Retention time from the admittance of the chlorine dioxide-nitrogen mixture to the unbleached pulp was 10 minutes. Consumption of chlorine dioxide was 1.10 percent of the dry weight of fiber. The pulp was then washed and treated at 20 percent consistency with 1.5 percent of its weight of sodium peroxide for 3 hours at 60 C. Characteristics of the semi-bleached pulp were brightness 69.5 and CED viscosity 25.7 cp.

EXAMPLE IV An unbleached Kraft pulp from coniferous wood having a Roe chlorine number of 4.8, a Kappa number of 31.2 and a 0.5 percent CED viscosity of 40.2 cps. was pressed to 30 percent consistency and comminuted into fibers and fiber aggregates.

One portion of the comminuted pulp was placed in a rotary glass vessel located in a bath maintained at C. Chlorine dioxide diluted to a partial pressure of 30 mm. of mercury with steam was passed into the rotary vessel until 1.5% C10 was passed through the pulp. Reaction time was 1 minute. The pulp was washed and the pulp treated at 20 percent consistency with 0.6 percent of its weight of hydrogen peroxide (100 percent basis) for 2 hours at 60 C. Characteristics of the Chlorine dioxide peroxide bleached pulp are set out in Table 1.

Another portion of the unbleached comminuted pulp was treated with gaseous chlorine at 32 C. for 1 minute; the pulp was then washed and treated with 3 percent of its weight of sodium hydroxide at 10 percent consistency for 90 minutes at 60 C. After washing, the pulp was treated with 1.5 percent calcium hypochlorite (as available C1 on 0D. pulp) at 10 percent consistency for 120 minutes at 40 C. Characteristics of the chlorinated, extracted and hypochlorite treated pulp are set out in Table 1.

TABLE 1 Characteristics Of The Semi-Bleached Kraft Pulp Chlorine Chlorine dioxidesodium hydroxide- Properties peroxide hypochlorite 0.5% CED Viscosity, cps 41.9 36.2 Pentosans, 7 6.8 6.7 Bleaching Yield on pulp, 71 98.2 93.7 Elrepho brightness, 7: 72.2 71.2

Physical properties at 300 CSF: Bulk, cc./g. 1.32 1.32 Breaking length, km. 13.3 13.6 Burst factor 114 Tear factor 136 137 We claim:

1. A process for bleaching an unbleached chemical cellulosic pulp which consists essentially of two successive stages, the first stage comprising adjusting the moisture content of said pulp such that said pulp has a consistency of from about 20 to 60 percent; shredding or fluffing the pulp into fibers and fiber aggregates; contacting said fiuffed pulp with gaseous chlorine dioxide diluted with a non-reactive gas to a partial pressure less than 100 mm. of Hg at a temperature between and 100 C. for a period of from seconds to 60 minutes and washing said treated pulp with water; and the second stage comprising adjusting the moisture content of said washed pulp such that said pulp has a consistency of from 5 to 50 percent; and then contacting said pulp with a peroxygen compound in a peroxygen content of from 0.025 1.5 percent by weight based on the weight of the dry pulp at a temperature up to C. for a period of from A to 5 hours.

2. A process as claimed in claim 1 wherein the peroxygen compound is selected from the group consisting of sodium peroxide, benzoyl peroxide, hydrogen peroxide and ozone.

3. A process as claimed in claim 1 wherein the pulp is contacted with the peroxygen compound at a consistency from about 10-30 percent.

4. A process as claimed in claim 1 wherein the'temperature of contact between said pulp and said chlorine dioxide is maintained by the exothermic reaction therebetween.

5. A process as claimed in claim 1 in which the pulp is a Kraft pulp whereby a semi-bleached pulp is obtained. 

2. A process as claimed in claim 1 wherein the peroxygen compound is selected from the group consisting of sodium peroxide, benzoyl peroxide, hydrogen peroxide and ozone.
 3. A process as claimed in claim 1 wherein the pulp is contacted with the peroxygen compound at a consistency from about 10-30 percent.
 4. A process as claimed in claim 1 wherein the temperature of contact between said pulp and said chlorine dioxide is maintained by the exothermic reaction therebetween.
 5. A process as claimed in claim 1 in which the pulp is a Kraft pulp whereby a semi-bleached pulp is obtained. 